1. Field of the Invention
The present invention generally relates to a direct broadcasting satellite receiver and more particularly is directed to a direct broadcasting satellite receiver capable of efficiently adjusting the receiving direction of its antenna for receiving a satellite broadcasting wave without using any special measuring apparatus.
2. Description of the Prior Art
Recently, a satellite broadcasting system has been realized which employs an artificial satellite existing on a stationary orbit to carry out television broadcasting.
A satellite broadcasting wave is an electrical wave in SHF (superhigh frequency) band, for example, about 12 GHz which comes from FM (frequency-modulation)--modulating a television signal.
FIG. 1 is a block diagram showing an example of a prior art receiving system of an SHF satellite television broadcasting wave.
An SHF signal transmitted is formed as follows. That is, a television audio signal is converted to a PCM (pulse code modulated) signal, and this PCM audio signal or data is divided into blocks at every predetermined time and added with an error correction code. Thereafter, the PCM signal is 4-phase-phase-shift-keying modulated on a subcarrier frequency of 5.73 MHz to become a signal which occupies the higher frequency side of an analog video signal as shown in FIG. 2. The modulated PCM signal and the analog video signal are combined with each other and then the composite signal is FM-modulated so as to become the SHF signal with the frequency of about 12 GHz.
This SHF broadcasting wave is received by a receiving parabola antenna 1 of the receiving system shown in FIG. 1. The signal of the SHF band received by the receiving antenna 1 is supplied to an outdoor unit or S-U converter 2 in which it is converted in frequency to a signal of UHF (ultra high frequency) band of, for example, about 1 GHz as a first intermediate frequency signal. The signal of UHF band is supplied through a coaxial cable 3 to a tuner 11 in an indoor unit 10. The tuner 11 receives from a channel-selecting circuit 12 a channel-selecting signal corresponding to a channel-selecting operation made by a viewer. As a result, this tuner 11 produces a broadcasting wave or signal (UHF signal) corresponding to the selected channel in the form of a second intermediate frequency signal. The second intermediate frequency signal is supplied through a band-pass filter 13 to a second intermediate frequency amplifier 14. The output from the second intermediate frequency amplifier 14 is supplied to an AGC (automatic gain control) voltage detecting circuit 15 which produces an AGC voltage. This AGC voltage is fed back to the amplifier 14 so that the amplifier 14 is controlled in AGC operation so as to make its output constant.
The output from the second intermediate frequency amplifier 14 is supplied to an FM demodulating circuit 16 and then FM-demodulated. The FM-demodulated output therefrom is supplied to a video amplifier 17 so that a video signal is developed thereby and delivered to an output terminal 18.
The output from the FM demodulating circuit 16 is also supplied to a band-pass filter 21 from which an audio subcarrier component is extracted. The audio subcarrier component thus extracted is supplied to a 4-phase-phase-shift keying signal demodulating circuit 22, in which the audio PCM signal is demodulated and which then is fed to a digital decoder 23. In this digital decoder 23, after the error of the PCM signal, which can be corrected by the error correction code, is corrected, the PCM signal is restored to the original time series and then re-converted to the analog audio signal. The analog audio signal from the digital decoder 23 is delivered to an audio output terminal 24.
The audio signal from the digital decoder 23 and the video signal from the video amplifier 17 are both supplied to an AM (amplitude modulation)-modulating circuit 19 which produces a signal modulated to a television broadcasting wave or signal of a particular vacant channel in VHF (very high frequency) band television broadcasting. This signal is delivered to an output terminal 20.
Accordingly, if the signal developed at the output terminal 20 is supplied to the antenna terminal of a home television receiver which is widely used, the satellite television broadcasting can be viewed by this television receiver at the particular vacant channel.
By the way, in the satellite broadcasting receiving system, the receiving parabola antenna 1 must be adjusted such that the direction thereof is directed so as to accurately face the artifical satellite. In this case, the adjusting of the direction of the receiving parabola antenna 1 requires high accuracy calculated down to first decimal place, for example, 30.5.degree.. The direction of the antenna is usually adjusted while inspecting the input level of the received signal. In this case, a special measuring apparatus is used because the adjustment of the receiving antenna must be carried out with high accuracy. In addition, when the direction of the antenna for receiving the satellite broadcasting wave is adjusted, it is necessary to adjust not only the azimuth angle thereof but also the elevation angle thereof. Thus, it takes much time to detect the optimum receiving direction of the antenna by merely measuring the level of the received signal and thus it is very troublesome to adjust the direction of the receiving antenna.